The present invention relates generally to filter mediums and more particularly to novel filter media suitable for use in filters such as oil filters and air filters of internal combustion engines.
The filter media used in the oil filters and the air filters of automotive vehicles must effectively remove or retain fine particles such as dust particles entrained in fluids requiring filtration or prefilts and must sustain the satisfactory filtration capability for a long period of time.
In view of the above requirements, there have been devised and demonstrated various types of filter media, but from the standpoint of a high filtration efficiency together with a long service life, they have not been entirely satisfactory in practice.
For instance, there has been proposed (for example, in Japanese Patent Publication No. 54-40778) a filter medium in which a plurality of filter paper sheets each having a different fiber density are laminated together so that the filter medium has a density gradient in the direction of the thickness thereof.
In this prior art density-gradient type filter medium as described above, the density is changed from coarse to dense from the upstream side to the downstream side thereof. As a result, relatively large particles are retained in a relatively low density layer while relatively fine particles are trapped in a relatively high density layer so that not only efficient and satisfactory filtration can be attained but also its service life is increased to some extent, but it has the following deficiencies or problems:
(a) With the prior art density-gradient type filter media of the type described above, the flow velocity of the feed fluid or prefilt passing through a filter medium is high. As a result, it is difficult to retain or trap dust particles by mere contact thereof with the filter medium and the dust particles are positively retained in the deep portion of the filter medium (the so-called filtration through the deep layer) so that the filter medium tends to be plugged or clogged very frequently, and consequently the pressure loss is increased.
(b) According to the results of extensive studies and experiments we have conducted, the porosity at the surface of a prior art filter medium on the upstream side of the feed fluid or "prefilt" is high, so that dust particles tend to be easily trapped in the surface layer, and consequently the formation of a dust cake layer upon the surface layer of the filter medium is adversely affected. As a result, the filtration through the cake layer (the so-called primal filtration) cannot be expected, whereby a long service life of the filter medium cannot be ensured.